In order to meet the requirement for lightweight vehicles, a method for replacing vehicle members conventionally formed by steel with lightweight aluminum alloy is employed. A material with its specific weight reduced to a steel ratio of 1/3 is used in general. As measures against a decrease in Young's modulus and material proof stress, as shown in FIG. 25, a closed section structure 20 or a closed section formed by laminating miscellaneous-shape members of a concave groove structure 30 and a plate 31 are used.
In case of mounting parts 11 and 12 on either side of a molded member 10 molded by conventional steel with open section, as shown in FIG. 26, parts 11 and 12 were installed on either side of the molded member 10, which were fixed with fasteners. As shown in FIG. 27, when an aluminum alloy closed section member 20 was introduced, the part 11 was fixed on one side of the aluminum alloy closed section member 20. So the other part 12 should be mounted on the other side wall 21 of the alloy closed section member 20. When the parts 11 and 12 are the members with heavy load, these connections are not proper to bear such heavy load.
When aluminum alloy extruded moldings were used for vehicle structural members, the conventional extruding method could not mold members with substantial changes in form. For this reason, aluminum alloy extrusion molded members used for automobile chassis and load-carrying platforms were members of a constant section form assuring a maximum stress area. As shown in FIG. 28, in case of constant section form member 40, section A is formed to bear the maximum stress. Though section B is loaded lightly, section B is formed to be the same to section A, resulting in over capacity and reducing efficiency of material use.
Furthermore, with a conventional steel chassis 50, a vehicle mainframe member 51 was disposed in layout shown in FIG. 29, while the use of an aluminum alloy closed section member 52 for the chassis resulted in disposition shown in FIG. 30, thus necessitating total modification of the layout.
With vehicle sashes for buses, etc., showcase and building sashes for which aluminium alloy extrusion molded members were used in various ways, a channel material, with a constant section, required after-machining of run-in in shaping up of sashes for visual correction and appearance modification thereof.
In this context, an extrusion molding method has been required for extrusion molded members with section forms varied along its extrusion axis.
As an extrusion molding method for extrusion molded members with section forms thereof changed along its extrusion axis, a method for manufacturing stepped tubes has been disclosed in the Japanese patent gazette Appl. No. H1-192,414. This method involves forming a mandrel head so as to have multiple steps, thereby allowing the mandrel head part partially different in diameter to go into and out of die holes, and thus manufacturing a tube 60 with a constant peripheral diameter with the inner wall thickness partially changed (Refer to FIG. 31).
Furthermore, a method for forming a protruding portion 71 in the periphery of an extrusion rod 70 shown in FIG. 32 has been disclosed in the Japanese patent gazette Appl. No. H2-48,324. This method involves forming a space 76 in the rear of a die hole 74 of a fixed die 72 shown in FIG. 33, usually forming by extrusion an extrusion rod 70 of the die hole 74, protruding, when forming the protruding portion 71, a mobile die 78, and filling billets in the space 76 forming the protruding portion 71.